There a number of paper pulp treating operations in which screens mounted on a reciprocal supporting conduit are disposed in a vessel. Thickening, displacement bleaching, and diffusion washing are three typical types of operations which are accomplished utilizing apparatus as typified by U.S. Pat. Nos. 3,704,603, 3,760,948, and 4,172,037 (the disclosures of which are incorporated by reference herein). While such techniques are imminently effective for treating pulp, they are not without their drawbacks.
One drawback that limits the versatility of such techniques is the fact that in many installations, in order to prevent the screens from clogging a volume of backflushing liquid is periodically introduced into the vessels. This causes a surge in the vessel volume which is unacceptable in multi-stage treatment techniques. Further, the equipment necessary to effectively perform the backflushing is expensive and complicated. Typically, an accumulator (which would be of titanium and thus very expensive, in the case of displacement bleaching), small pump for filling the accumulator, two on-off valves, and a controller for the valves, are necessarily provided in prior art systems. Even so, the backflushing liquid is supplied to the vessel at less than optimum conditions since the accumulator typically supplies the backflushing liquid at a velocity that decreases from the start of application to the completion. Further, the constant opening and closing of the extraction and backflushing valve, and the resulting disruption on the treatment process, restricts the efficiency and capacity of the treatment.
According to the present invention, the above-mentioned drawbacks are overcome in a simple yet effective manner. According to the present invention, the backflushing liquid is supplied from within the vessel itself, rather than from the exterior of the vessel. The backflushing liquid is typically provided, according to the invention, from a cylinder which is mounted to the bottom of the screen supporting conduit. The cylinder has a first open end in communication with the interior of the screen support conduit, and a second open end in open communication with the pulp being treated within the vessel. A piston is mounted within the cylinder, and relative movement between the piston and cylinder is effected. Utilizing this apparatus, it is possible to entirely eliminate the accumulator and its associated pump, and to replace the two on/off valves and their associated controller with a simple control valve. Thus extraction can be continuous rather than intermittent, and the extraction volume is proportioned with respect to the effective volume of the cylinder so that efficient backflushing occurs.
Utilizing the invention, not only is it possible to eliminate the accumulator, piping, and its other associated components as described above, but it is also possible to more efficiently treat a larger capacity of pulp. For example, according to the invention it is possible to increase capacity of pulp being treated by as much as about 15% in the case of diffusion washing, while increasing the efficiency of the diffusion washing by as much as 20 to 30%. Since the entire volume of backflushing liquid is provided from within the vessel itself, surge is eliminated thereby expanding the number of treatment techniques that can be practiced utilizing the invention.
Also according to the invention, a method of effecting treatment of pulp with screens while minimizing screen clogging is provided, utilizing a generally upright vessel having a pulp inlet and a pulp outlet, the screens mounted to a supporting conduit within the vessel, and the screens having screening surfaces extending generally vertically. The method comprises the steps of: (a) Passing pulp generally vertically in the vessel from the inlet to the outlet. (b) Effecting substantially continuous extraction of liquid from the screens through the screen support conduit at a predetermined flow rate. (c) Effecting generally vertical movement of the screens and support conduit in the direction of pulp flow at a first speed, and in the direction opposite pulp flow at a second speed much higher than the first speed. (d) When moving the screens in the direction opposite pulp flow, supplying a known quantity of liquid to the conduit and screens from within the vessel to effect backflushing of the screens without a surge in the vessel volume. And, (e) controlling the continuous extraction flow rate with respect to the known quantity of backflushing liquid so that the known quantity of backflushing liquid is sufficient to effectively backflush the screens to prevent clogging thereof. The method according to the invention is particularly adapted to (although not restricted to) diffusion washing, thickening, and displacement bleaching.
A significant object according to the present invention is to effectively back flush screens in a pulp treatment vessel in a simple manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.